The present invention relates generally to a Light Emitting Diodes (LEDs) and, more specifically, to a circuit and method for flicker suppression in LEDs.
Recent developments of high-brightness light emitting diodes (LED) have opened new horizons in lighting. Highly efficient and reliable LED lighting continuously wins recognition in various areas of general lighting, especially in areas where cost of maintenance is a concern.
A wide dynamic range of the LED brightness control becomes important in many applications, such as automobiles, avionics and television. In some cases it is needed due to large variation in the ambient light, in others it allows to improve the contrast ratio of a display. Due to the color and chromaticity properties of LED's, it is beneficial to control brightness of an LED through pulse width modulation of the current in it, while maintaining the current magnitude at a fixed level. This LED brightness control method is commonly referred to as the PWM dimming.
Referring now to FIG. 1, one example of a PWM dimming scheme in a prior art LED driver of the boost type is shown. The boost converter power train (hereinafter boost converter) in the FIG. 1 includes an inductor 103 receiving input power from an input voltage source 101 via a power switch 102, and delivering power to an output filter capacitor 106 and an LED string 107 via a rectifier diode 105.
The brightness control circuit of the boost converter of FIG. 1 includes a PWM switch 108 receiving a brightness control signal from a PWM pulse generator, the PWM switch 108 periodically disconnecting the LED string 107 from the output of the boost converter when the output of the PWM pulse generator 100 is low. The brightness control circuit also includes an LED current sense element 109; an error amplifier 110 having a reference IREF and a compensator network 112; a hold switch 111 for disconnecting the compensator network 112 from the output of the error amplifier 110 when the output of the PWM pulse generator 100 is low; a peak current sense element 104 for detecting peak current in the inductor 103; a current sense comparator 115 for comparing the output of the current sense element 104 with an error voltage at the compensator network 112, and for generating a reset signal when the error voltage is exceeded; a PWM latch turning the power switch 102 on upon receiving a clock signal 117, and turning the switch 112 off upon receiving the reset signal; a logic gate 118 for inhibiting the turn on of the switch 102 when the output of the PWM pulse generator 100 is low.
When the PWM dimming pulse 100 becomes very small (less than one switching cycle for a DCM design or less than a few switching cycles for a CCM design), the boost converter is turned off before the input current can reach its steady state value. This may cause the input power to droop, which is manifested in the output as a droop in the LED current.
Referring to FIGS. 1 and 2, the inductor 103 current 203 begins with the first clock pulse 117 and 217 following the leading edge of the dimming pulse 100 and 200. The inductor 103 current 203 is not given enough time to reach the reference 212 generated by the error amplified 110, and the output current control loop opens.
Therefore, it would be desirable to provide a circuit and method that overcomes the above problems.